Nonwoven products comprise loosely assembled webs or masses of fibers bound together with an adhesive binder. Adequately bonded nonwoven fabrics have advantages over woven fabrics for a large variety of uses. It is known to form bonded nonwoven fabrics by impregnating, printing or otherwise depositing an adhesive bonding composition on a base web of fibers. These fibers may be of cellulosic or polymeric materials such as wood pulp, polyesters, polyamides, polyacrylates and the like. The base web of nonwoven fibers, to which the binder is applied. can be produced by carding, garnetting, air laying, wet laying, paper making procedures, or other known operations.
The polymeric binder must imbue the bonded nonwoven product with acceptable dry and wet tensile strengths and solvent resistance for the intended application.
One of the more successful copolymer binder compositions for nonwoven products comprises a vinyl acetate/ethylene/N-methylolacrylamide copolymer. (See U.S. Pat. No. 3,380,851). However, such N-methylolacrylamide (NMA) containing copolymers liberate formaldehyde during cure and subsequent use of the nonwoven.
U.S. Pat. No. 4,449,978 discloses nonwoven products bonded with a binder comprising a polymer of vinyl acetate/ethylene/N-methylolacrylamide/acrylamide. These nonwoven products have a low residual free formaldehyde content.
Other comonomers which have been utilized in preparing self-crosslinking polymer binders include, in addition to N-methylolacrylamide (NMA), N-iso-butoxymethyl acrylamide (IBMA).
U.S. Pat. No. 3,730,933 discloses stable vinyl acetate/N-methylolacrylamide copolymer emulsions containing fully-hydrolyzed polyvinyl alcohol prepared by dispersing the polyvinyl alcohol in water substantially in the absence of the monomeric reactants and effecting copolymerization while adding both monomers gradually and simultaneously to the polyvinyl alcohol dispersion, the vinyl acetate and the NMA being added in substantially constant ratio and at a rate substantially not exceeding the rate of their copolymerization.
U.S. Pat. No. 4,446,274 discloses a vinyl acetate-ethylene copolymer emulsion, the copolymer also comprising an acid monomer and a N-alkoxymethyl acrylamide. The vinyl acetate-ethylene copolymer emulsion may be obtained by a two-step process comprising the first step of polymerizing 20-80% of vinyl acetate monomer under an ethylene atmosphere until the degree of polymerized monomers reaches 5-50% of the total monomers, and the second step of adding a mixture containing the remaining portion of vinyl acetate monomer, an acid monomer and a N-alkoxymethyl acrylamide monomer followed by completion of polymerization under the ethylene atmosphere.
U.S. Pat. No. 4,448,908 discloses a latex, the particles of which comprise a polymer core and a shell thereover, the shell comprising a water insoluble monomer of the formula ##STR1##
U.S. Pat. No. 4,497,917 discloses latex compositions comprising core-shell polymer particles.
Application Ser. No. 714,661 filed 21 March 1985 discloses binder polymers of formaldehyde-free crosslinking monomers. Illustrative of such formaldehyde-free crosslinking monomers is acrylamidobutyraldehyde diethyl acetal (ABDA). The problem was how to make the most efficient use of the new functional monomers such as ABDA in vinyl acetate emulsion polymers. Continuous addition of these monomers throughout the reaction as a substitute for NMA on equivalent basis provided polymers with encouraging properties but they still fell short of tensile strengths of the NMA-containing copolymer. The ABDA monomer being expensive would not provide an economical product if additional equivalents of ABDA were added, so a more efficient use of the ABDA monomer already present had to be developed.
Conventional techniques developed for preparing polymers containing NMA and IBMA-type monomers do not promote efficient use of the ABDA-type monomers.